[WIP] new s3 scanner

I implemented a S3 scanner that iterates over all objects in a bucket and updates the filecache so we have an up to date mtime for syncing. It fixes https://github.com/owncloud/core/issues/8633 when the file scan for the user is triggered periodically.

Keep in mind that amazon s3 users pay for the traffic, so don't execute it every 5 min.

It is still WIP as the Scanner became S3 specific and I need opinions on the approach. cc @icewind1991 @schiesbn @PVince81 

Still think this is the only solution. Also: browsing the web interface ALWAYS makes a trip to the bucket. we could use that to update the filecache as well, but it only helps if someone browses the directory tree.